Recently, thanks to the development of techniques processing external image signals, it becomes possible to use a TV as a monitor for a PC by setting an external input mode of the TV to a PC input mode and connecting an image signal output line of the PC to a PC input terminal among input terminals of the TV. That is, it is possible to display a PC operating screen on the TV. At this time, as the PC operating screen displayed on the TV, there is a window screen. In these operating screens, the window operating screen has a vertical frequency of 60 Hz.